Metal fatigue is a problem common to just about any component or structure that experiences cyclic stresses or repetitive loading. Such problems are especially important in the metal structures utilized in various components of transportation systems as they experience a varying amount of repetitive loads during normal operation. Structures or components that are prone to fatigue damage include, but are not limited to, commercial and private transport aircraft, general aviation, military aircraft, helicopters, jet engines, turbines, passenger cars, trucks, off-road equipment, construction vehicles, heavy construction equipment, boats, ships, trains, rolling stock, railroad track, stationary and moving bridges, medical implants, pressurized pipes and vessels, guns, cannons and the like.
Metal fatigue can generally be defined as the progressive damage, usually evidenced in the form of cracks, that occurs to structures as a result of cyclic or repetitive loading. The lower surface of an aircraft wing is a classical example of the type of loading that produces fatigue. The wing is subjected to various cyclic stresses resulting from gust, maneuvering, taxi and take-off loads, etc., which over the service life of the aircraft can produce fatigue damage.
Fatigue damage is generally observed, at time of initiation, in the form of growth of small cracks from areas of highly concentrated stress. Typical stress concentrators include holes, fillet radii, abrupt changes in section, notches, and the like. Fatigue damage can often be hidden to the untrained because it generally occurs under loads that do not generally cause yielding or deformation of the structure. In fact, failure usually occurs under loads typically experienced in the operation of the structure. Undetected, a fatigue crack can grow until it reaches a critical and catastrophic size or length. At the critical length, the unstable crack races through the metal, causing sudden failure of the component. Catastrophic failure of the entire structure, such as a wing or fuselage, can occur when other members of the structure can not carry the additional load from the failed member.
Even stationary objects such as railroad track, pressurized vessels and artillery equipment may fail in fatigue because of cyclic stresses. Cyclic loads caused by repeated loading due to rail car wheels running over an unsupported span of railroad track are the cause of many track failures. In fact, some of the earliest examples of fatigue failures were in the railroad and bridge building industry. Sudden pressure vessel failures can also be caused by repeated pressurization cycles acting on initially small cracks. It is not surprising that U.S. governmental studies report that fatigue damage is a significant economic factor in the U.S. economy.
While many methods have been developed and utilized for the manufacture of structures having improved fatigue life at fasteners, it would nevertheless still be desirable to reduce the amount of handling involved in producing such structures. That is because such a development would facilitate reduced manufacturing costs of enhanced fatigue life structures, thus reducing the cost of end products utilizing such structures, and/or enabling more widespread use of improved fatigue life components in industrial applications.